PART II. DIGITAL HALFTONING FUNDAMENTALS

Transcription

1 PART II. DIGITAL HALFTONING FUNDAMENTALS Outline Halftone quality Origins of halftoning Perception of graylevels from halftones Printer properties Introduction to digital halftoning Conventional digital halftoning methods - Block replacement - Screening (dithering) + clustered-dot + dispersed-dot - Error diffusion (briefly mentioned) II-1

2 REFERENCES ON FUNDAMENTALS OF DIGITAL HALFTONING [1] J.P. Allebach, "Visual model-based algorithms for halftoning images," in Image Quality, Proc. SPIE 310, , [2] J.P. Allebach, editor, Selected Papers on Digital Halftoning, SPIE Milestone Series, vol. MS 154, [3] J.F. Jarvis, C.N. Judice and W.H. Ninke, "A survey of techniques for the display of continuous tone pictures on blievel displays," Computer Graphics & Image Proc., 5, 1-40, Mar [4] P.R. Jones, "Evolution of halftoning technology in the United States patent literature," J. Electronic Imaging, 3, , July [5] K. Knowlton and L. Harmon, "Computer-produced grey scales," Computer Graphics & Image Proc., 1, 1-20, [6] F. Nilsson, "Halftoning and objective quality measures for halftoned images," Linkoping Studies in Science and Technology Thesis No. 671, [7] W.F. Schreiber, Fundamentals of Electronic Imaging Systems, Springer-Verlag, [8] J.C. Stoffel and J.F. Moreland, "A survey of electronic techniques for pictorial image reproduction," IEEE Trans. Commun., 29, , Dec [9] R. Ulichney, Digital Halftoning, MIT Press, II-2

3 HALFTONE QUALITY HALFTONE FEATURES IMAGE REGIONS halftone-induced textures grayscale (tonescale) resolution grayscale (tonescale) accuracy adjacent graylevel compatibility spatial resolution constant graylevel slowly changing graylevel rapidly changing graylevel (edges) II-3

4 HALFTONE-INDUCED TEXTURES Examine halftoning of constant graylevel patches. Good halftone-induced textures, i.e. pleasing patterns: - Avoid large white gaps and large black clusters. pp. I-5, II-5: rand. thresh. halftoning large gaps & clusters - Avoid low frequency energy. pp. II-6 through II-8: examples of halftone-induced textures and their spectra: - Have their energy in high frequencies, i.e. "blue noise". p. II-9: Ulichney's ideal bluenoise spectra - Have less energy in horizontal and vertical than in diagonal. p. II-10: left column -- diagonal screening (4 8 classical) right column -- rectangular screening (6 6 Bayer) - Lack unusual or annoying artifacts. pp. I-8, I-10 and p. II-11: ramp halftoned with four methods: from left -- 4x8 classical screening, 6x6 Bayer screening, bluenoise screening, Fl.-St. error diffusion In some cases, induced texture quality varies with graylevel. - Low frequency energy varies with graylevel because dot spacing varies. p. II-12: halftone-induced textures for two different gray levels, with error diffusion - Artifacts may vary with graylevel. p. II-11: ramps halftoned with four methods II-4

5 RANDOM THRESHOLD SCREENING II-5

6 HALFTONE-INDUCED TEXTURES Graylevel = 1/8 on 0 to 1 scale, 0 = black, 1 = white. Halftoning by "error diffusion" with two different filters. Induced Textures Radial Spectra Plotted are 2-dimensional spatial frequency spectrum averaged around a circle with radius equal to frequency. ρ θ = printer resolution in dots per degree From R. Ulichney, Digital Halftoning, MIT Press, 1987 II-6

7 SPECTRA OF HALFTONE-INDUCED TEXTURES Graylevel = 1/8. Halftoning by "error diffusion" with two different filters. Induced Textures Radial Spectra From R. Ulichney, Digital Halftoning, MIT Press, 1987 II-7

8 SPECTRA OF HALFTONE-INDUCED TEXTURES Graylevel = 1/8. Error Diffusion Induced Textures Random Threshold Screening Radial Spectra From R. Ulichney, Digital Halftoning, MIT Press 1987 II-8

9 ULICHNEY'S IDEAL BLUENOISE SPECTRUM From R. Ulichney, Digital Halftoning, MIT Press 1987 II-9

10 Diagonal Screening (8x8 classical) (6x6) Rectangular Screening II-10

11 Classic screen Bayer screen Bluenoise scrn Error diff'n (8x8) (6x6) (F-S) II-11

12 HALFTONE-INDUCED TEXTURE MAY VARY WITH GRAYLEVEL Error diffusion applied to two different graylevels graylevel = 1/8 gray level = 1/16 Induced Textures Radial Spectra Very light patterns and very dark patterns will necessarily have more low frequency energy From R. Ulichney, Digital Halftoning, MIT Press, 1987 II-12